This invention relates to mechanisms for rapid insertion of a neutron absorbing poison into the core of a liquid cooled nuclear reactor. In order to increase the safety of nuclear reactors, particularly the liquid-metal cooled type, it is desirable to augment the regular plant protection system (PPS), whereby electrical signals initiate the insertion of poison rods into the reactor core, with self-actuated scram (SAS) devices that initiate a scram in direct response to potentially dangerous values of some system parameter. It is known that a SAS in response to a low coolant flow rate incident can be achieved by supporting the bundle containing the poison rods with coolant pressure differentials acting on a hydraulic latch plug such that a reduction in flow rate will cause a reduction in differential pressure across the plug, resulting in the poison bundle release and insertion into the core under the influence of gravity. Despite the straightforward safety response of this mechanisms to the loss of flow incidents, such prior art mechanisms have two shortcomings. First, these prior SAS mechanisms are responsive only to loss of flow incidents. Second, even for the loss of flow incident, the response time for the poison to enter the core is too slow to assure that no fuel will be damaged in the reactor core.